techniques based on molten materials is usually referred to as fused
deposition modelling (FDM) even though this is the name of the
filament-based approach [2, 4]. As different technologies, still
based on melt extrusion, were developed over time, a new nomenclature was recently suggested. In particular, the pressurized gas/
piston-based approach is now being referred to as 3D-fiber deposition, while bioextrusion is used for screw-driven processes [5].
Although each approach has its own advantages, the type of
material that can be used with a particular technique is limited by
the manufacturing process. For example, FDM and SLS require
polymers that become moldable above a specific temperature and
solidify upon cooling (called thermoplastics), SLA requires the use
of a photopolymer resin, and 3DP involves the use of solvents and
binders [6, 7].
Tissue engineers have focused their attention on AM techniques attracted by the possibility of controlling the geometry of the
scaffold, an essential parameter to be considered when mimicking a
tissue’s morphology and mechanical properties. Principles and
materials typically used in industry have been adapted to the stricter
requirements of tissue engineering, mainly due to biocompatibility,
but also to the different product morphologies involved, less dense
and with a high level of porosity.
Although the first machines developed starting from the 1950s
were based on SLA, FDM techniques are probably the most successful ones due to their simplicity and low demands on hardware,
which makes low-cost printers possible [8]. This applies to the field
of tissue engineering in particular, where the high costs of medical
grade materials and of the process toward clinical application are
pushing research groups to use extrusion-based machines. With the
only theoretical requirement for the material to be extrudable, the
devices are usually simpler and the operational risks lower compared
to SLA, SLS, and 3D printing. This is due to the fact that, generally,
no toxic precursors or binders are needed during the shaping
process. Additionally, when working with polymers as for hard
tissue engineering (e.g., bone and cartilage), melt-based techniques
are preferred to solution-based extrusion as they do not involve any
solvent that might be toxic for cells. As the polymer cools down and
solidifies after extrusion, its stiffness and mechanical strength are
high enough so that no curing/gelation step is needed (usually
needed with hydrogels). The polymer also retains the given shape as
no solvent evaporates.
In tissue engineering, extrusion techniques have been successfully used to produce scaffolds made of hydrogels as well as thermoplastic polymers. The use of hydrogels allows the user to avoid
solvents or high temperatures for the extrusion, thanks to their
viscoelastic behavior. Therefore, tissue engineers have started
embedding cells in the material to be extruded to avoid the further
step of cell seeding. Additionally, a better controlled spatial
AM of Thermoplastics for Tissue Engineering
77
deposition modelling (FDM) even though this is the name of the
filament-based approach [2, 4]. As different technologies, still
based on melt extrusion, were developed over time, a new nomenclature was recently suggested. In particular, the pressurized gas/
piston-based approach is now being referred to as 3D-fiber deposition, while bioextrusion is used for screw-driven processes [5].
Although each approach has its own advantages, the type of
material that can be used with a particular technique is limited by
the manufacturing process. For example, FDM and SLS require
polymers that become moldable above a specific temperature and
solidify upon cooling (called thermoplastics), SLA requires the use
of a photopolymer resin, and 3DP involves the use of solvents and
binders [6, 7].
Tissue engineers have focused their attention on AM techniques attracted by the possibility of controlling the geometry of the
scaffold, an essential parameter to be considered when mimicking a
tissue’s morphology and mechanical properties. Principles and
materials typically used in industry have been adapted to the stricter
requirements of tissue engineering, mainly due to biocompatibility,
but also to the different product morphologies involved, less dense
and with a high level of porosity.
Although the first machines developed starting from the 1950s
were based on SLA, FDM techniques are probably the most successful ones due to their simplicity and low demands on hardware,
which makes low-cost printers possible [8]. This applies to the field
of tissue engineering in particular, where the high costs of medical
grade materials and of the process toward clinical application are
pushing research groups to use extrusion-based machines. With the
only theoretical requirement for the material to be extrudable, the
devices are usually simpler and the operational risks lower compared
to SLA, SLS, and 3D printing. This is due to the fact that, generally,
no toxic precursors or binders are needed during the shaping
process. Additionally, when working with polymers as for hard
tissue engineering (e.g., bone and cartilage), melt-based techniques
are preferred to solution-based extrusion as they do not involve any
solvent that might be toxic for cells. As the polymer cools down and
solidifies after extrusion, its stiffness and mechanical strength are
high enough so that no curing/gelation step is needed (usually
needed with hydrogels). The polymer also retains the given shape as
no solvent evaporates.
In tissue engineering, extrusion techniques have been successfully used to produce scaffolds made of hydrogels as well as thermoplastic polymers. The use of hydrogels allows the user to avoid
solvents or high temperatures for the extrusion, thanks to their
viscoelastic behavior. Therefore, tissue engineers have started
embedding cells in the material to be extruded to avoid the further
step of cell seeding. Additionally, a better controlled spatial
AM of Thermoplastics for Tissue Engineering
77